most powerful between 9:00 a.m. and 3:00 p.m., so those times form the left and right edges of the solar window frame. The second diagram shows how the frame is defined for a theoretical location.

The solar window is important because more than 80% of the available energy comes through it. If there is anything in the way of that energy, your solar installation will run at reduced efficiency, so you need to make sure that nothing blocks that window. Even a bare tree's branches will interrupt and diffuse available sunlight, reducing the available radiation by as much as 75%.

Of course, you can't do anything about the Earth's atmosphere and all the dust and dirt it contains, but you can ensure there are no man-made obstacles in the way – buildings, masts, aerials, landscaped trees or whatever.